This blog discusses current weather, weather prediction, climate issues, and other topics

Tuesday, January 6, 2009

Heavy rain in mountains, but rainshadowed Puget Sound.

It is an interesting observation that the heaviest precipitation in the mountains and worst flooding in area rivers can be associated with relatively dry, balmy weather in Seattle and the central PS lowlands. To get heavy, unusual precipitation in the mountains you need a good upward component along the western mountain slopes. If the winds are mainly southerly the flow is mainly parallel to the mountains (except for the southern Olympics) and you don't get big-time orographic rainfall. But westerly winds are another story....strong westerly winds, particularly with a warm subtropical origin, can give massive amounts of precipitation...often rain...that can add up to 10-20 inches over a few days. And that is what is going to happen over the next 48h hrs. The irony of this, is that westerly winds produce a nice rain shadow east of the Olympics that can protect Seattle and neighboring regions. That is why Seattle will get wet, but not flooded. To show you what I mean, take a look at a recent latest radar image (attached). Look how dry it is over central PSound. But if you head east you hit the rain by North Bend.The precipitation is now mainly over the northern portion of the State, but the current of moist flow will settle south this afternoon. I have attached the model forecast 24-h precipitation for two periods...ending 4 AM tomorrow and Thursday morning. One is struck by two things...the tremendous rainfall in the mountains (reds are 5-10 inches). Clearly, some mountain locations will get 10-15 inches. The other feature is the rainshadow over Puget Sound...particularly today. (Note..the first figure is from a higher resolution simulation). You can also see a rainshadow to the lee of the mountains of Vancouver Island over Georgia Strait. The air coming into the region will be very warm, with freezing levels rising to 6-7 thousand feet. The snowpack can absorb some of the water, but plenty will go into the rivers. That plus the melting of the huge low-level snowpack has led the NWS to put up a flood watch for many of the local rivers. If you live in an area that often floods...you should prepare...now.

18 comments:

Anonymous
said...

Hi Cliff,

What's your opinion of the NWS's forecast table interface with regard to the precip forecast? I notice that the 6-hr rainfall forecast can vary considerably several times a day. Do you know the basis of the forecast?

Wow, we are getting VERY strong gusts in Ravenna (NE Seattle). There are branches down all over, the tree by my son's school (Woodland Park) has 3 large limbs torn off and its garbage-can alley in all the side roads where bins are empty. The phone lines were down at school too.

There's some pretty fascinating cloud movement and wind over downtown right now (sorry, I know I'm not speaking in proper terminology). There's a very low level of clouds that are moving very quickly to the ENE, and a slightly higher, more solid level of clouds moving almost perpendicular to the SSE. Really neat stuff!

Centralia Chehalis newpaper reporting one of the NWS models is predicting all time records for Newaukum and Skookumchuck rivers, 3 feet above the 06 Cowlitz flood in Randle, and one foot below our 07 Chehlis flood which was an alltime event.

Years ago (before I retired) I worked on a meteorological measurement system. We installed a unit on the building for marketing purposes. I just checked and it's still there. I know you have lots of data and instrumentation available to you, but if you ever want some real-time data for Redmond, here you go.

Funny, I thought I would be bored with this blog once the snow event ended, but I'm still enjoying it.... plus, now I crave extreme weather, guiltily hoping for really windy weather and a good power outrage.... sadly, I live on an east-facing slope in the lee, whah.

I too am hoping for major rain. We still have lot's of snow here east of North Bend to get rid of. I have to abandon my car at the end of the driveway and walk in. The river is coming up slowly (Middle Fork Snoqualmie) but nothing like '06. I hope it doesn't get that bad. Getting some good gusts in North Bend proper, but I think our place is somewhat sheltered. I was on the 520 bridge earlier. First time I have ever felt it actually rockin' and rollin'!

Cliff, I appreciate your detailed yet layman's understandability approach; I find myself checking your blog on a daily basis now and am struck at your accuracy and reliability. My brother-inlaw works for one of the BIG Seattle TV stations and I find myself anylizing your predictions vs. the BIG 7. Keep up the good work; do you work as a meterologist yourself?-THE WATCHER